1. Field of the Invention
This invention relates to a metal gasket for chain case-carrying engines, wherein combustion chamber bore-carrying thin metal plates are laminated so as to seal with this laminate a narrow clearance between opposed surfaces of a chain case-carrying cylinder block and a cylinder head.
2. Description of the Prior Art
A conventional metal gasket for chain case-carrying engines is adapted to maintain in an air- or liquid tight state a narrow clearance between the upper surfaces of a cylinder block and a chain case, which are formed unitarily in an adjacently positioned state, and a lower surface of a cylinder head, and provided with primary seal portions which require a high seal surface pressure and auxiliary seal portions which require only a comparatively low seal surface pressure. An auxiliary seal portion of a low seal surface pressure is formed by extending at least one of the laminated thin metal plates, which constitute a primary seal portion, as a base member, and then bonding a soft sheet, such as a .beta.-cellulose sheet, a graphite sheet or a compressed sheet to one surface or both surfaces of this extended base member. The known metal gaskets of this kind include the gaskets disclosed in Japanese Utility Model Laid-Open Nos. 48059/1988, 27857/1991 and 27858/1991.
In the metal gasket disclosed in Japanese Utility Model No. 27857/1991, a hole is made in the portion of a thin metal plate which is opposed to a height different part-carrying upper surface portion of a cylinder block or a height different part-carrying lower surface portion of a cylinder head so that the hole crosses a bead formed around a through bore, and a seal member is provided on the inner side of the hole with a part of the seal member projecting therefrom.
In an OHC type or DOHC type engine, a chain case 29 having a hollow space 33 in which a cam shaft driving chain moves is generally provided independently of and adjacently to a cylinder block 28 as shown in FIG. 11, and it is combined by bolts with the cylinder block 28 which is provided with cylinders, i.e. combustion chambers 35 therein. In this engine, a metal gasket is provided between the cylinder head and chain case 29 and the cylinder block 28. The cylinder head and chain case 29 are fixed to each other by bolts inserted in bolt holes 32. In this engine, a mounting side surface of the chain case 29 is formed in an opened state so as to reduce the weight of the chain case 29. The hollow space 33 is defined by an inner surface of the chain case 29 and a side surface of the cylinder block 28.
This engine has a structure in which separately formed parts are fixed to each other as mentioned above. Therefore, even when an upper surface 34 of the cylinder block 28 and that 31 of the chain case 29 are finished flat, it is unavoidable that height difference 30, though they are very small, occur between these upper surfaces at abutting surfaces 20 in a boundary between the cylinder block 28 and chain case 29, and that the height differences 30 become larger after the engine is used for a long period of time. When the height differences 30 occur between these upper surfaces, an oil, such as a lubricating oil is splashed, while a chain for driving a cam shaft moves in the chain case 29, over the height-differences and enters the chain case therefrom, the resultant oil leaking from the height-differences to the outside to enter a narrow space between the opposed surfaces of the cylinder block 28 and cylinder head.
Such a metal gasket formed by extending a thin metal sheet out of laminated thin metal sheets, which is positioned on the side of the cylinder block 28, as a base member, and bonding a soft sheet on one surface thereof, the air-tightness and liquid-tightness cannot be maintained due to the height differences 30. Such a metal gasket formed by extending an intermediately positioned thin metal plate as a base member, and bonding soft sheets to both surfaces thereof can maintain the air-tightness and liquid-tightness when the height differences 30 are extremely small but, when the height differences 30 become larger, the increasing of the thickness of the soft sheet is required so as to secure the sealing performance, and it becomes necessary to design the lamination of metal plates by determining the thickness of a primary seal portion taking the thickness of the soft sheet into consideration. In general, a .beta.-cellulose sheet, a graphite sheet or a compressed sheet which constitutes the soft sheet has an original thickness of at least around 0.5 mm, so that it is necessary to set the thickness of a primary seal portion accordingly. In order to secure a high surface pressure at the primary seal portion, the number of thin metal sheets to be laminated is necessarily increased.
In this metal gasket, a hole is formed along a circumference of a through bore, and a seal member is provided on the inner side of the hole so that a part of the seal member projects from the hole. Around the circumference of the through bore, the sealability of portions opposed to the height different parts is maintained by the seal member provided on the inner side of the hole. Moreover, this seal member extends along the whole of the circumference of the through bore, and is not fitted in an intermediate plate.
In general, the height differences occurring between abutting surfaces of a cylinder block and a chain case include height differences occurring when the upper surface of the chain case is lower than that of the cylinder block, and height differences occurring when the upper surface of the chain case is higher than that of the cylinder block. When the upper surface of the chain case is lower than that of the cylinder block, the surface pressure of the chain case decreases with respect to the cylinder block, so that the surface pressure of the height different parts and chain case has to be taken into consideration. When the upper surface of the chain case is higher than that of the cylinder block, the surface pressure is concentrated on the chain case to cause the surface pressure of the cylinder block to decrease. The surface pressure of the circumference of a combustion chamber bore in the cylinder block requires to be at a high level, and, when a metal gasket is used, the regulating of a surface pressure becomes difficult.
Therefore, when height differences occur between the flat continuous lower surface of the cylinder head in this metal gasket and the upper surfaces of the cylinder block and chain case therein, the problem of preventing entry of an oil into the height different parts in the chain case has to be solved without taking a means for increasing the number of the thin metal plates in the laminate which causes the gasket manufacturing cost to increase greatly.